Acoustic technique-facilitated research on the evaluation of the population dynamics and formation of spawning ground for northern Japan Sea walleye pollock stock (Doctoral thesis)

 

Summary of the thesis

Objectives

Walleye pollock, Theragra chalcogramma is an important fishery resource and was specified as a target species by the total allowable catch (TAC) system when the system was implemented in 1996. Since the system conducts quantitative management, acoustic resource assessment – a resource estimation method independent of fishery – gained significant importance.

The walleye pollock around Japan is divided into four stocks. The northern Japan Sea stock, the subject of this research, is caught mainly by the offshore trawl fishery, the gill net and longline fishing in the Japan Sea, Hokkaido. In the gill net and longline fishing, adult walleye pollock approaching shore for spawning are caught between November and March. The catch in this sea area declined from 120,000 t in 1991 to 25,000 t in 2005, with the resource status and recruitment per spawning biomass (RPS) at the lowest level ever.

In recent years, influence of environmental change on fisheries resources has caught attention. There have also been reports that describe negative correlation between surface water temperature in winter and recruitment of the northern Japan Sea walleye pollock stock. However, except for information on spawning season, there is little ecological information available on how walleye pollock is responding to the environmental change.

An environmental change observed in the Japan Sea is an increase in the number of years in which there have been strong Tsushima Warm Currents, whose surface layer flow to the north, since 1988.  It is now clear that the coastal water of Hokkaido has a warming trend according to the data over 100 years.

On the other hand, the distribution of walleye pollock with water temperature shows that walleye pollock are sometimes caught in the water at 12˚C when they are feeding, while they are distributed in water with temperature as low as 0.5˚C in their spawning season. The optimum water temperature for the eggs to hatch is 2–7˚C. The hatchability decreases at water temperature higher than 10˚C.

In order to properly manage the stock of walleye pollock, which is declining significantly, it is necessary to accurately evaluate the resource volume and to immediately clarify the influence of environmental change on the resource. Therefore, this research aimed to compare the virtual population analysis (VPA) estimates of walleye pollock with its biomass obtained in the acoustic resource survey, and to clarify the influence of water temperature on the spawning ground formation, which is the first step in the reproduction process of walleye pollock, by analyzing the distribution and the water temperature information obtained from the acoustic resource survey.

 

Materials and Methods

Acoustic resource surveys were conducted in the Japan Sea, Hokkaido during September–October period between 1996 and 2007. The survey used the research vessel Hokuyo Maru (237 t) throughout the survey period, with Kinsei Maru (151 t) accompanying as well from 2001 onward. An echosounder EK500 or EK60 (Simrad) was used for recording acoustic data. The frequency and pulse duration used in the echosounders were 38 kHz and 1 ms, respectively. Before starting the survey, the echosounders were calibrated as much as possible using a calibration sphere (60 mm diameter, copper sphere).

To examine the reliability of the recorded acoustic data, calibration records of the echosounders were analyzed and inter-vessel calibration was performed between Hokuyo Maru and Kinsei Maru. Also, the effects of three types of removal processing for background noise using EchoView were evaluated. Afterward, biomass of spawning adult walleye pollock calculated by applying the correction factor from this study was compared with resources estimate calculated by VPA.

Also the vertical distribution of water temperature at the two fixed points (Iwanai Bay and off Hiyama) was compared with the depth distribution of walleye pollock obtained from the acoustic resource survey. To verify the obtained relationships, correlation between the catch of coastal fisheries since 1935 and the normalized water temperature calculated from the coastal water temperature data of the Japan Sea, Hokkaido was analyzed. Furthermore, the correlation between the new recruitment since 1981 obtained by VPA and to RPS was analyzed similarly.

Lastly, to confirm the location where the current spawning ground is formed, egg distribution survey was conducted using three research vessels in the coastal Japan Sea in February 2006 and 2007. Eggs were collected by vertical haul with NORPAC or 80R net from 150 m in depth. The Assessment Reports of the Tomari Nuclear Power Plant (Hokkaido, 2006 and 2007) were used to obtain the data of the coastal water in Iwanai Bay.

 

Results and Discussion

1. Evaluation of acoustic resource survey using echosounder　(Chapter 2, 3)

The calibrated values of echosounders in Hokuyo Maru showed an annually declining trend. Of these, the value 25.34 dB in 1998 was found to be abnormally low compared with the values in the years of order. Although the data collected by the echosounders in Kinsei Maru was 0.66 - 1.10 higher than that of Hokuyo Maru, in recent years a high concordance of 0.95-fold or higher has been seen. Background noise removal using the three processing methods gave good concordance (0.8 - 1.2-fold) with the results obtained by manual processing.

The biomass estimated by the acoustic resource survey decreased from 254,000 t in 1999 to 88,000 t in 2007. The biomass had a declining trend throughout the survey period and gave a significant correlation coefficient of r = 0.886 when compared with the VPA (except 1996, 1997 and 2002). When the absolute values were compared with the VPA values, it was in the range of 0.9–1.5-fold concordance with a mean value of 1.3-fold, and variation coefficient of the difference was approximately 80%. This research indicated that there is a possibility that the VPA underestimated the fluctuation range of resources due to the error of age determination. However, the highly correlated estimates obtained from two completely different estimation methods indicate the reliability of each method. Reliable resource evaluation was considered possible by complementing the disadvantage of one method with the advantage of the other.

 

2. Distribution of walleye pollock in spawning migration season and water temperature (Chapter 4)

The center of depth distribution of walleye pollock between 1996 and 2006 was in the range of 385–485 m in Iwanai Bay and 375–445 m off Hiyama in 1996–2006. Diurnal vertical migration of a shoal of walleye pollock was also observed. The water temperature at the center of the distribution was between 0.53 and 0.98˚C, with 2005 being the only year in which the temperature exceeded 0.8˚C. Furthermore, depth distribution of walleye pollock showed a high correlation of r² = 0.656, with the water temperature in the layer of 100 m in depth (except 2005). The reason as to why the fish swam in such a low water temperature layer is to reduce the energy loss before spawning.

 

3. Catches of spawning walleye pollock and coastal water temperature (Chapter 5)

The catches in Shiribeshi Sub-prefecture showed a significantly negative correlation with the normalized water temperature between January–March period and October–December period. Conversely, in Hiyama Sub-prefecture, the catches showed a significantly positive correlation with the water temperature in January and December. The reason as to why the catches declined in Shiribeshi Sub-prefecture, which has high water temperature and located in northern part, is assumed to be because the depth distribution of walleye pollock becomes deeper during the high water temperature years, which causes the spawning shoal going to the spawning ground in Shikotan Peninsula and northward that have relatively shallower sea floor depth to decrease; instead, the spawning shoal went to the ground off Hiyama.

The recruitment per spawning (RPS) of walleye pollock showed significantly negative correlation with water temperature in winter (December to March). Furthermore, the relationship between RPS and water temperature appeared to be divided into two phases: low water temperature–high RPS years and high water temperature–low RPS years.

 

4. Change in location of the spawning ground for the northern Japan Sea stock (Chapter 6)

There is indication that the spawning ground of walleye pollock off western Hokkaido in the northern part of the Sea of Japan has disappeared. This is inferred from recent reduction in the fish catch by the coastal fisheries, which capture spawning adults. In the present research, the distribution of the walleye pollock eggs was investigated over the western Hokkaido, Sea of Japan in February 2006 and 2007. In Ishikari Bay, Stage1 eggs’ maximum density was actually small: 4 individuals/m², and the eggs were not collected in Soya and Rumoi region. However, in the southward from Shakotan Peninsula, Stage1 eggs’ maximum density was 19,350 individuals/m² in Iwanai Bay, and 1,574 individuals/m² in Hiyama region. These suggest that Iwanai Bay and Hiyama region (off Otobe) was the major spawning ground, and Ishikari Bay was a minor one. However, there was no spawning ground in Soya and Rumoi region recently. Ishikari Bay has many eggs transported but has few eggs produced. We assume that some eggs spawned off Otobe, Hiyama region, would be transported northward, and only a small amount southward, and that the location of the main spawning ground changed after 1980s by comparison with this result and past knowledge. As a result, the distance between main spawning grounds and nursery area became larger in recent years. Consequently, the eggs could encounter more often high water temperature, which is fatal to the development. It was thought that these didn’t allow successful transport of the eggs to the nursery area.

 

5.  Influence of water temperature on the spawning ground formation and the survival in the egg stage (Chapter 7)

The results above indicate that the spawning ground in the area in Ishikari Bay and northward is currently decaying due to the high water temperature in the season of spawning migration (October) . The current climate regime is causing the low RPS, and the high and low RPS within this condition could be explained by the water temperature in the spawning season (February) in the area from Iwanai Bay to the coast off Hiyama and by the transport of eggs and larvae. The hatchability is reduced when the surface water temperature increases to 7 to 8˚C and higher. The survivability is reduced when the eggs and larvae are not transported to the northern coastal area. In conclusion, in order for eggs to survive successfully, eggs and larvae have to be transported to the wide nursery ground located in Ishikari Bay and northward, while maintaining high hatchability at the same time.

Ecological and physiological studies on masu salmon Oncorhynchus masou masou smoltification and applied technology for masu salmon stock enhancement (Doctoral thesis)

 

Masu salmon Oncorhynchus masou masou is an important fish not only for the coastal fisheries off the northern part of Japan, but also for recreational fishing in inland waters and the coastal waters off northern Japan. On the other hand, masu salmon stocks have been decreasing since the 1980s. Although the main reason is loss of the riverine habitat due to human activity, another cause is the lack of information on the biological features of wild populations for the improvement of masu salmon stocks. Objectives of this study are to examine the movement and growth of wild masu salmon juveniles, to define the ecological and physiological process of the smoltification of wild populations, to show the effect of external factors (water temperature and daylength) and internal factors (genetics) on the smoltification and to discuss the mechanism of smoltification, and to develop smolt release technology for stocking.

Eighteen stream populations from Hokkaido and Honshu in Japan, and one strain from the domesticated masu salmon were used for the field examination and rearing experiment from 1981 to 1996.

The diel behavior of emerging fry from the experimental incubator showed greater activity at nighttime than in the daytime during the early period of emergence. Fry from the eyed egg deposition under the three stream beds distributed mainly at 1 km downstream and were dispersed to 8 km downstream.

Individual marking release-recapture surveys were conducted to investigate the Specific Growth Rate (SGR) for body length of wild juveniles. The SGRs held at high levels in the first spring to summer and decreased in autumn, and rapidly increased in the following spring.

Wild juveniles with an individual tag in the upper and middle reaches of the Atsuta River showed that the middle reach group were larger in size, and had a higher SGR and a low remaining ratio in autumn. The middle reach group was considered to be the main component of smolt. Comparing the large parr group with the small group in the SGRs from the two populations, there was a significant difference in SGRs in the Shimonaehutoro River in autumn but not in the Yobetsu River in summer. The peak of smolt migration was earlier in the southern streams from May to early June than in the northern streams in June.

The developmental process of smoltification in wild juveniles was morphologically classified into six stages: the yellowish parr in winter and early spring, the silvery parr in autumn and spring, the presmolt, the midsmolt, the fullsmolt and the small parr in spring. Biological features of the smoltification, SGR, serum thyroid hormone (T4) concentration and seawater adaptability were tested on wild juveniles in smolting. The SGRs increased from the beginning of smoltification at the stage of the presmolt. Although serum T4 levels were at a high level during the smoltification, seawater adaptability showed a high quality at the stages of the midsmolt and the fullsmolt. The number of chloride cells on gill lamellae from the southern wild population histologically began to increase in March but the height of the follicle cells in the thyroid gland increased from April. The peak of seawater adaptability coincided with the peak of the seaward migration of smolt. Although there was no clear tendency in the deal fluctuations of serum T4 from migrating smolt, the surge of serum T4 was observed in one stream after rainfall. The surge was considered to depend on the turbidity from the rainfall. Annual fluctuations in the composition of the smolting stage were observed in one stream and showed that water temperature affected the development of smoltification of wild juveniles.

The SGR, smolt timing, smolting ratio, seawater adaptability and serum T4 were examined under artificial conditions to show the effect of water temperature, daylength (photoperiod) and population genetics on the smoltification of masu salmon. The results showed that a high water temperature (8℃) promoted smolting but a low water temperature (2℃) inhibited smolting in the wintering wild juveniles. A short term daylength (8L16D; daytime for 8 hours and nighttime for 16 hours) treatment did not inhibit smolting in this season. The two siblings of juveniles under the three constant daylengths (16L8D, 8L16D, 4L20D) at 8℃ WT from emergence to smolting showed that the long term daylength (16L8D) treatment inhibited smolting. On the other hand, the modified three daylength experiments (14L10D, 12L12D, 10L14D) under an 8℃ WT with transferring on a differential season (July, August, September, October, December and January) indicated that the 14L10D treatment in July, August and September inhibited smolting, and although the 12L10D and 10L14D treatments in July, August and September were 100％ smoltfied (except 10L14D on July), the smolting ratio decreased thereafter. Therefore, the it juvenile response for daylength had changed between September and October, and the critical daylength for the response was located between 14L (daytime for 14 hours) and 12L (daytime for 12 hours). It is strongly suggested that the critical point might be the autumnal equinox.

Because the juveniles from the three stream populations indicated the inherent change of smolting features under the same rearing conditions, it was shown that the smoltification of the population was genetically controlled. Moreover, it was confirmed that the F1 hybrid had an intermediate peak of smolt timing between the two offspring from each parent.

The results indicate that the smoltification of wild masu salmon is controlled by the following smolting process and mechanism. Underyearing juveniles are inhibited from smolting due to a long daylength (more than 14L) in summer and they arrive at the critical point in daylength between 14L and 12L in autumn. It is likely that the autumnal equinox turns on a genetic switch of smoltification. The low water temperature (less than 2℃) inhibits the advance of smolting in winter and the juveniles develop the smoltification with genetic smolt run timing due to the rising water temperature in the following spring. On the other hand the southern populations in Honshu and Kyushu, Japan, which consist of only the riverine form, are considered to have turned on the maturation switch regardless of smolting at the autumnal equinox.

The experimental smolt release with large (13.6cm, 27.6g) and small (12.9cm, 21.0g) smolt groups and the ecological coastal research showed the effect of large smolt on survival and the difference in feeding ecology between the large and small smolt. The large smolt group had a higher survival (0.165%) from the release to adult return in the river than the small group (0.067%). Although the two smolt groups fed on sand lance Ammodytes personatus larvae in coastal waters, oceanic zooplankton was mainly available for the large smolt. It was confirmed that the large smolt release was better than the small smolt among the same populations.

This study showed the high growth rate at the presmolt stage in smolting and the surge of serum T4 of migrating smolt after the rainfall. There are considered useful indicators of the evaluation for smolt quality and release timing. Therefore, it is important that we monitor not only coastal environments, but also the growth rate of juveniles and stream conditions. And careful monitoring of the relationship between the smolt and other organisms in coastal waters is also required in order to observe the influence of smolt cannibalism.

Fishing management of the red snow crab in the sea of Japan off southern Hokkaido

 

Experimental fishing of the red snow crab is being conducted off the coast of southern Hokkaido in the Sea of Japan. Little is known about a number of aspects of the population dynamics of this stock, including the somatic growth of individuals, natural mortality, recruitment, and emigration, which are important factors in its fishing management. In this paper, the advantages and disadvantages of several management strategies for this stock were evaluated using an operating model. A management scenario in which the annual TAC changed according to the annual change in CPUE caused a large fluctuation in the annual catch. A scenario in which the TAC was maintained over several years according to the trend in CPUE kept catch fluctuations at a low level, although a higher level of fishing effort led to a greater risk of failure in  management. In such cases, failure of management could be avoided by placing an upper limit on fishing effort; however, a large variation emerged in the cumulative catch obtained from the stock. The calculated output of various scenarios greatly differed according to the level of fishing effort at the start of the management scenario. Application of these results to practical fishing management for this stock requires estimates of current fishing effort levels and stock number trends to be as close to reality as possible.

Homing of masu salmon in the tributaries of the Shiribetsu River evaluated by returns of marked fish

 

Homing of masu salmon was evaluated by examining the number of marked fish returning to tributaries of the Shiribetsu River in 2007.  Marked masu salmon spawners were recovered at a high rate (80.5%) in the Mena River where most of the marked juveniles had been released into, but were at lower rates (4.3%) in the other 10 tributaries, indicating that the homing ability of masu salmon among tributaries is high.  Only a few marked fish were recovered in tributaries where hatchery-reared fish had not been stocked.  Those marked fish were stocked as smolts, and a portion of smolts was transported and released in the lower reach of the mainstem Shiribetsu River.  Such release history may be a cause of the straying into the tributaries.

Morphological and biochemical early development of wakasagi based on wild-caught samples from Lake Abashiri, Hokkaido, Japan

 

This report describes the morphometric changes and the biochemical development presumed by RNA, DNA and protein contents associated with the development in wakasagi based on wild samples from Lake Abashiri. Inflection points on the body proportion were recognized at 12, 33, 42, and 54 mm total length, corresponding to the phase transitions approximately. Early biochemical development of waksagi based on the DNA base indices would consist of three cytological stages, namely, the cell proliferation period (hyperplasia, Phases A-E except D), the cell elongation period (hypertrophy, Phases F-G), and the higher level period than both of the proliferation and elongation periods (Phases D and H). The biochemical development showed steps similar to its morphological development, with the changes coincident to both. Since a shift of the feeding habit is seen just before the transition, the morphological and biochemical developments would be mutually related to the shift, and are suggested to occur in the same phases.

Densities of spawning redds of chum and masu salmon observed in an urban river (Short Paper)

 

Densities of spawning redds of chum and masu salmon in the Izari River, a tributary of the Ishikari River, which flows in an urban area were surveyed in September 2008.  A total of 440 redds of chum salmon and 94 redds of masu salmon were observed within a reach of 9 km long; the mean densities within the survey reach were 4.9 redds / 100 m for chum salmon and 1.0 redds / 100 m for masu salmon, respectively.  Conservation of the naturally spawning salmon populations and their spawning habitat in such an urban area is important.

Distribution of masu salmon spawning redds in the Gunbetsu River, Bishabetsu River and Gokibiru River in Hokkaido (Short Paper)

 

The distribution and number of spawning redds of masu salmon Oncorhynchus masou in the three non-conservation rivers (Gunbetsu River, Bishabetsu River and Gokibiru River) in Hokkaido were investigated, and the results were compared with the previous studies of the conservation river. One hundred and sixty-five redds per 9.1km in the Gunbetsu River, twenty redds per 2.0km in the Bishabetsu River and one hundred and thirty three redds per 9.2 km in the Gokibiru River were found, respectively. The number of redds per km in these non-conservation rivers was equal to the neighboring conservation river of the Atsuta, indicating the importance of non-conservation rivers for the spawning of masu salmon.

A gigantic lacustrine sockeye salmon Oncorhynchus nerka (kokanee or hime-masu) captured in Lake Toya, Hokkaido, Japan (Short paper).

 

An extremely large kokanee was captured in Lake Toya, Hokkaido, Japan. Since the previous two records of Tanakadate(1925, 1940), this is the third record of a gigantic kokanee specimen in over half a century.

Biological characteristics, distinction and identification of Ascidiella aspersa（Müller, 1776）, as an alien ascidian in northern Japan（Technical report）